Screening the Surface Structure-Dependent Action of a Benzotriazole Derivative on Copper Electrochemistry in a Triple-Phase Nanoscale Environment

Copper (Cu) corrosion is a compelling problem in the automotive sector and in oil refinery and transport, where it is mainly caused by the action of acidic aqueous droplets dispersed in an oil phase. Corrosion inhibitors, such as benzotriazole (BTAH) and its derivatives, are widely used to limit such corrosion processes. The efficacy of corrosion inhibitors is expected to be dependent on the surface crystallography of metals exposed to the corrosion environment. Yet, studies of the effect of additives at the local level of the surface crystallographic structure of polycrystalline metals are challenging, particularly lacking for the triple-phase corrosion problem (metal/aqueous/oil). To address this issue, scanning electrochemical cell microscopy (SECCM), is used in an acidic nanodroplet meniscus|oil layer|polycrystalline Cu configuration to explore the grain-dependent influence of an oil soluble BTAH derivative (BTA-R) on Cu electrochemistry within the confines of a local aqueous nanoprobe. Electrochemical maps, collected in the voltammetric mode at an array of >1000 points across the Cu surface, reveal both cathodic (mainly the oxygen reduction reaction) and anodic (Cu electrooxidation) processes, of relevance to corrosion, as a function of the local crystallographic structure, deduced with co-located electron backscatter diffraction (EBSD). BTA-R is active on the whole spectrum of crystallographic orientations analyzed, but there is a complex grain-dependent action, distinct for oxygen reduction and Cu oxidation. The methodology pinpoints the surface structural motifs that facilitate corrosion-related processes and where BTA-R works most efficiently. Combined SECCM–EBSD provides a detailed screen of a spectrum of surface sites, and the results should inform future modeling studies, ultimately contributing to a better inhibitor design.


S.1 ADDITIONAL EXPERIMENTAL DETAILS
Pipette pulling parameters. All pipettes were pulled to a size of approximately 400 nm diameter with the following pulling parameters:

S.2 OXIDE LAYER REDUCTION CALCULATIONS
Polished Cu surfaces develop an oxide layer upon exposure to air, with a thickness and composition that depends on the exposure time and atmospheric conditions. A thin layer of Cu(I) oxide, Cu2O (of approximately 2.5 nm) 1-2 forms immediately after polishing, and a further layer of Cu(II) oxide, CuO, grows on the top of it with slower kinetics (i.e., thickness of ≈5 nm after 2 months exposure). 3 Under the present conditions (the sample being analyzed one day after the polishing procedure and stored under ambient conditions), a native oxide layer (denoted CuOx, herein) of approximately 2-3 nm can reasonably be assumed. Assuming an oxide layer thickness of 3 nm and taking density and molar mass values of 6.0 g cm −3 and 143.09 g mol −1 , respectively for Cu2O and; 6.31 g cm −3 and 79.545 g mol −1 , respectively for CuO; charge densities of ≈ 2.5 mC cm −2 and ≈ 4.6 mC cm −2 can be calculated for Cu2O and CuO, respectively. Thus, 2.5 mC cm −2 is taken to be the lower limit of charge required to remove a mixed-valence CuOx film of thickness 3 nm from Cu. It is further possible to estimate the number of "layers" of oxide considering that Cu2O has a cubic crystallographic structure, with a lattice constant a ≈ 4.26 Å, with each cell containing effectively 4 Cu atoms, arranged in an fcc sublattice. Therefore, taking the (001) surface as a model, there will be a layer containing two coplanar Cu + ions every half of the lattice constant, i.e. ≈ 2.13 Å. Assuming that a layer of Cu2O is coincident with a layer of Cu + ions, in a 3 nm oxide layer, there will be approximately 14 Cu2O monolayers, corresponding to a charge density of ≈ 0.2 mC cm −2 per monolayer.
In the present study, SECCM meniscus contact was made at − 0.45 V vs. Ag/AgCl/3.4 M KCl (Ag/AgCl hereafter) and held for a period of 0.25 sec before linearly scanning the potential anodically, with the substrate (working electrode) current measured throughout. Median i−t curves, obtained during the initial landing period, with and without BTA−R, are shown in S5 Figure S3. Initially (t < 5 ms), both curves present a sharp near-exponential decay, followed by a "shoulder" before entering a region where i ∝ t −1/2 , roughly after 0.1s. Assigning the first exponential region to non-faradaic processes (e.g., double layer charging and stray capacitance from the measurement set up) and the final region to the cathodic reactions relevant to corrosion (discussed in the main text), the "shoulder" is assigned to the (partial) reduction of the native oxide layer. Integrating over this region produces median values of 0.63 and 0.62 mC cm −2 without and with BTA−R, respectively, which corresponds to approximately 3-4 monolayers of Cu2O (vide supra). This is a fraction of the charge that would be required to fully reduce the native oxide layer (2.5 to 4.6 mC cm −2 , assuming 3 nm thick layers of Cu2O or CuO, respectively). 4 This analysis indicates that the native oxide layer is converted into a complex, mixed layered structure, consisting of an electro-reduced Cu(0) overlayer that protects the underlying CuOx from further reduction, giving rise to a Cuo|CuOx|Cub sandwich structure ("o" and "b" refer to overlayer and bulk Cu, respectively), similar to what has previously been proposed by Nakayama et al.

S6
Interestingly, the calculated oxide reduction charge appears to be grain dependent, as shown by the maps in Figure S2, respectively for the case without ( Figure S2a Figure S8c, (c) Figure   4f and (d) Figure S8f.     Figure S7f are overlapped onto each frame of the movie. S11

S.5 GRAIN ORIENTATION-2D PROJECTION
The development of the grain orientation 2D projection employed in this work has been previously described in detail. 6 Here a brief recall is given for clarity of understanding. The average orientation of each crystallographic grain present in the scanned areas (measured through EBSD, detailed in section S.1) was extracted as Euler angle data, φ1, Φ and φ2. The average miller indexes (h,k,l) for each plane were calculated from Φ and φ2: h=n sin Φ sin φ 2 (S1) The miller indexes for each grain were sorted from smallest to largest, for symmetry reasons.
(h',k',l', such as h' ≤ k' ≤ l') and were used to calculate the angles between any generic plane α and each of the three low-index planes employed for the cubic system representation, (001), (011), (111), respectively as γ1, γ2 and γ3: Therefore, the orientation α could be expressed with a point P of coordinates (γ1 α , γ2 α , γ3 α ). as well as their model 3D representations, are shown in Figure S6. S14 Figure S9. Definition of the grain ID for each grain analyzed from (a) Movie S1 (i.e., reproduction of the EBSD data shown in the main text, Figure 4c) and (b) Movie S2 i.e., reproduction of the EBSD data shown in the Supporting information, Figure S7c).   Figure S7f).  Figure S11. Correlation plot for the grain orientation spread (GOS) for the inhibitor-free case.